Re: [Intel-gfx] [PATCH v2 1/4] drm/i915/gt: GEM_BUG_ON unexpected NULL at scatterlist walking

[Matthew Auld <matthew.auld@intel.com>]

On 25/04/2022 17:24, Ramalingam C wrote:
> While locating the start of ccs scatterlist in smem scatterlist, that has
> to be the size of lmem obj size + corresponding ccs data size. Report bug
> if scatterlist terminate before that length.
> 
> Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_migrate.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c b/drivers/gpu/drm/i915/gt/intel_migrate.c
> index 9d552f30b627..29d761da02c4 100644
> --- a/drivers/gpu/drm/i915/gt/intel_migrate.c
> +++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
> @@ -687,6 +687,12 @@ static void get_ccs_sg_sgt(struct sgt_dma *it, u32 bytes_to_cpy)
>   		bytes_to_cpy -= len;
>   
>   		it->sg = __sg_next(it->sg);
> +
> +		/*
> +		 * scatterlist supposed to be the size of
> +		 * bytes_to_cpy + GET_CCS_BYTES(bytes_to_copy).
> +		 */
> +		GEM_BUG_ON(!it->sg);

It will crash and burn anyway, with the below NULL deref. Not sure if 
BUG_ON() is really much better, but I guess with the additional comment,
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

>   		it->dma = sg_dma_address(it->sg);
>   		it->max = it->dma + sg_dma_len(it->sg);
>   	} while (bytes_to_cpy);